Split render script for VSE?
Use the parallel render add-on: https://github.com/elmopl/ktba/tree/master/scripts/addons

Install it.
Add an absolute path to ffmpeg in the add-on preferences.
Find the Parallel Render options in the Render sidebar.

(For really fast render use AVI JPEG and PR, but you'll have to render audio separate afterwards and mux it yourself)